This is a bit tongue in cheek, but I suspect Apple has benefitted from having smarter users. I have no science bhind this opinion, but I suspect that more affluent and richer people have been the first adopters.
If Apple should be so lucky as to gain parity in market share, it will include some of the same folks who click on everything and install everything. It will then attract the same kind of unscrupulous software vendors that MS attracts.
;^)>
You know what's funny? I just finished reading a comment on one of the other websites reporting this WebApplication report where the commenter—obviously a Windows Fanboy—made the argument that Apple was intentionally targeting the stupidest 10% of the computer market. His arguments included choosing a computer with no software, that it couldn't read files from Windows computers, no peripherals that worked with it other than Apple made, the one-button mouse, the high cost compared to better PCs, and that the Mac was really crash prone. It was self evident, he reasoned, that if they were smart they would buy Windows computers.
However, be it either way, the Mac OS pretty much prevents stupidity in installing something. First of all, on Mac OS X, jumping the execution pointer into the data stack with the intent of executing malicious code hidden in data or buffer overflows from stuff gotten off the web or clicked on just won't work—the data stack and heap memory locations are non-executable. Secondly, standard users cannot install software, or alter the system software, without knowing and entering an administrator name and password. Thirdly, it takes more than a single or double click, even for an administrator, to download and install something from the Internet. A user has to first accept a download of an executable, even if it's hidden inside an image or a codec, then agree to install the application including giving it an administrator name and password, and then again, give it explicit permission to run for the first time. You have to be almost industrial strength stupid to do all of that unconsciously.
On the other hand, I have met some industrial strength stupid people.